1. Field of the Invention
The present invention relates to the seaming of plastic fabrics into endless loops for use as industrial belts in the production of paper, board and similar products. More specifically, the invention concerns the use of ultrasonic energy to produce a seam by welding together the overlapped ends of a length of plastic fabric to form an endless belt.
2. Description of the Prior Art
Ultrasonic energy has long been used to weld or bond plastic elements to one another. The prior art in the textile and industrial fabric fields alone indicates that many attempts have been made to use ultrasonic energy to join plastic yarns and fabrics together. In the latter case, the uses have been both to join lengths of plastic fabric into endless forms to produce an endless belt or ribbon, and to patch holes in such fabrics by covering them with pieces of identical fabric and welding the pieces in place over the holes.
For example, U.S. Pat. No. 3,184,354 to Strother shows a method for splicing yarns, particularly multifilament yarns of a synthetic polymeric resin, by means of pressure and high frequency (ultrasonic) vibrations. The method results in a small splice of substantially circular cross section, usually smaller than the normal free diameter of the yarn. More importantly, the splices are such that the dye characteristics of the yarn portions joined in the splice are unchanged, so that the splices remain inconspicuous in the finished product. To accomplish this desirable effect, the yarns are compressed in a slot in a die by a tool, which applies ultrasonic energy to the compressed yarns, whereby the fibers or filaments thereof are softened, compacted and securely bonded to one another without producing at the splice point a solid mass that would not accept dye in the same manner as does the yarn.
U.S. Pat. No. 4,018,955 to Klauke et al. shows a continuous printing ribbon constructed by overlapping the ends of a length of ribbon made of thermoplastic synthetic material and welding the overlapped ends together along a grid work of fine weld lines. This provides a weld joint comprising a punctiform arrangement of ink-bearing cells to effect an undisturbed emission of ink across the joint during a printing operation. The conditions of pressure and temperature under which the grid-like network of fine line welded joints are formed are produced ultrasonically, whereby the overlapped ribbon end portions are "hammered" at an ultrasonic rate, the result of which is to generate sufficient heat in the ribbon material to fuse the impacted portions thereof.
U.S. Pat. No. 4,090,897 to Minick shows a method and apparatus for patching or seaming thermoplastic mesh fabrics, such as those used in the papermaking industry, by means of an ultrasonic welding tool. The tool is provided with one or more tips having raised surface portions or projections adapted for contacting the plastic mesh at a plurality of spaced points whereby ultrasonic welding is accomplished without substantial disruption or diminution of the generally uniform porosity inherent to the mesh fabric. The method modifications involve the use of templates for guiding the positioning and movement of the ultrasonic welding tips for both butt seaming and overlap seaming of the opposed edges of a thermoplastic mesh fabric. In the butt seaming technique, the outermost weft filaments of the opposed edges of the plastic fabric are replaced by one heat-stabilized filament and the picket ends of the fabric are interdigitated about this stable filament preparatory to the welding step. In any case, when the ultrasonic tool is pressed downwardly against a thermoplastic material, such as a mesh fabric superimposed on another fabric made of the same thermoplastic material, the vibratory energy causes localized heating between the two fabric layers and the thermoplastic will consequently melt and flow to form a permanent welded bond of the two superimposed fabric layers in a fraction of a second. Lap seams are shown to be made by spot-welding the overlapped fabric edge.
U.S. Pat. No. 4,490,199 to Dunning shows an apparatus and method of splicing polymeric webs using an ultrasonic welder and a web support means. Means are provided for securing the trailing end of at least one first web and the leading end of at least one second web between the ultrasonic welder and the web support means. The trailing end of the first web and the leading end of the second web overlap between the ultrasonic welder and the web support means and are welded together. Preferably, the ultrasonic welder heats the polymeric material in the polymeric web to a temperature above the melt temperature and exerts a pressure on the webs causing them to fuse together.
U.S. Pat. No. 4,501,782 to Weatherly et al. shows a method for bonding thermoplastic webs. The method includes the steps of interdigitating the ends of the warps at opposing web edges, wherein the length of the interdigitation insures that the end of an interdigitated filament overlies the first shute of the opposing web. The interdigitated fibers are subsequently ultrasonically bonded in a continuous manner across the width of the web so that the ends are crushed down and flattened over the corresponding shute. The warp ends may overlie a number of opposing shutes as long as the overlap is no more than 11/2 times the thickness of the web. The web bonding method for producing a continuous belt produces a web joint which is substantially coplanar with the remainder of the web such that the web is uniquely adapted to papermaking processes which avoids bumps or ripples in the sluice that solidifies on the web. In a further embodiment, ultrasonic energy is utilized to bond the web edges.
In general, the use of ultrasonics to seam plastic fabrics, either by overlapping or butting the opposite ends thereof, into endless form for use as belts in the paper production and related industries, has not met with any noteworthy success. The principal reason for past failures in this respect may be traced to the effect that ultrasonic welding has on the plastic material itself. When the yarns, such as monofilament yarns, from which industrial fabrics may be woven, are extruded from molten polymeric resin materials, they are subjected to a sequence of drawing and relaxation which extends the constituent molecules and aligns them with respect to one another. Among the properties given to the yarns during these steps in their production are linear strength and flexibility.
In the past, the seaming of plastic fabrics by ultrasonics has been by welds produced by the melting of the polymeric material of the yarns at the weld point, and by the fusing and subsequent solidification of the liquid polymeric material into a solid mass. Inevitably, this fusing and subsequent solidification of the polymeric material at the weld changes its structural properties relative to those of the yarns. Most importantly, the crystallization which occurs on solidification makes the weld more brittle than the yarns themselves. As a consequence, when placed under longitudinal tension and when flexed about rolls, possibly of small diameter under such tension, the seam has the tendency to break at the relatively inflexible weld.
With the present invention, the Applicant has provided an improved method for forming an overlapped seam in plastic fabrics using ultrasonics. The seam produced is stronger and flexible, does not have the brittleness and consequent early failure rate of prior-art seams joined by ultrasonics, and may be formed in one hour, as compared to the 80 hours typically required to form a woven seam in fabrics of this type.